Method, composition and product with improved adhesion between a metal member and a contiguous rubber skim stock

ABSTRACT

This invention is directed to a method, a rubber skim stock and a product containing the skim stock having improved adhesion between a metal member and contiguous rubber skim stock. The invention lies in the discovery that improved rubber-to-metal adhesion can be obtained by adding to an otherwise conventional rubber skim stock composition appropriate amounts of either p-aminobenzoic acid or a cobalt salt of p-aminobenzoic acid. 
     The method of this invention comprises the steps of mixing the p-aminobenzoic acid or cobalt salt of p-aminobenzoic acid into a rubber composition, bringing this composition into contiguous relationship with a metal member in an unvulcanized product and vulcanizing the product to yield the end product.

RELATED APPLICATIONS

This application is a continuation-in-part composite of our copendingapplications, U.S. Ser. No. 824,767 and U.S. Ser. No. 824,780, bothfiled on Aug. 15, 1977 now abandoned.

BACKGROUND OF THE INVENTION AND PRIOR ART

In the production of rubber articles such as hose, pneumatic tires orpower transmission belts such as V-belts, toothed positive drive belts,etc., it is generally necessary to reinforce the rubber or elastomericproduct. In the past, textile materials have been employed for thispurpose. However, wire cord has been found to be more desirable undercertain conditions of use, for example, in pneumatic tires of the radialply type. Maximum reinforcement of the rubber is obtained when maximumadhesion is produced and retained between the laminate of a contiguousrubber skim stock and the metal reinforcing element as used to form aunitary structure. Of equal importance is the requirement that, forexample, the laminate of the reinforcing metal element and contiguousrubber remain in a bonded relationship with each other throughout theuseful life of the reinforced structure in which the laminate is used.

To achieve satisfactory rubber-to-metal bonding, numerous methods havebeen developed. For example, U.S. Pat. No. 2,720,479 describes a systemwherein a phenolic resin and a brominated isoolefin-polyolefininterpolymer are dissolved in a suitable liquid carrier and theresulting adhesive composition is spread on rubber which is to be bondedto metal. The rubber and metal are subsequently pressed together andvulcanized. U.S. Pat. No. 2,581,920 also describes the use ofhalogenated polymers to bond rubber to metal.

In U.S. Pat. No. 3,517,722 to Endter et al., a rubber-metal adhesionsystem is described which involves formation of aresorcinol-formaldehyde resin at the interface between the rubber andthe metal, thereby bonding these members together. In forming the resin,compounds capable of liberating the methylene precursor are added toresorcinol in the rubber stock. Upon vulcanization, the methylene andresorcinol react to form the resorcinol-formaldehyde resin.

According to U.S. Pat. No. 3,846,160, the adhesion force between steelcord and rubber is enhanced by applying a mineral oil solutioncontaining, for example, an organic acid salt of a higher aliphaticamine to zinc plated or brass plated steel cord.

The adhesion of rubber to metal such as wire tire cord is improvedaccording to U.S. Pat. No. 3,847,727 by incorporating a halogenatedquinone and the condensation product of resorcinol and acetaldehyde intothe rubber prior to application to the metal and vulcanization of thecomposite.

The disclosure of U.S. Pat. No. 3,903,026 teaches the preparation of arubber composition having improved metal adhesion properties even afterthermal aging; this is disclosed to be achieved by compounding cobaltcarboxylate and magnesium oxide (0.1 to 4 phr) into rubber.

U.S. Pat. No. 3,738,948 is directed to a fiber reinforced rubbercomposition that can be utilized in tire construction. The fiber can beglass, nylon, rayon or metal wire. The composition is disclosed tocontain a finely divided silica, hexamethylene tetramine, resorcinol anda compatible metal soap such as calcium stearate. Similar compositionscontaining discontinuous fiber filaments are disclosed in U.S. Pat. No.3,746,669.

U.S. Pat. No. 3,340,214 teaches the use of benzoic acid or nitrobenzoicacid as an additive to reduce the resiliency of rubber as utilized, forexample, in the production of tires.

DESCRIPTION OF THE INVENTION

This invention relates to a method, a rubber skim stock and a productcontaining the skim stock wherein the invention yields improved adhesionbetween a contiguous rubber skim stock and a metal member. The inventionlies in the discovery that improved rubber-to-metal adhesion can beobtained by adding to an otherwise conventional rubber skim stockcomposition appropriate amounts of either p-aminobenzoic acid (PABA) ora cobalt salt of p-aminobenzoic acid.

The method to this invention comprises the steps of mixing the PABA or acobalt salt of PABA into a rubber composition, as described below,bringing this composition into contiguous relationship with at least onemetal member in an unvulcanized product and vulcanizing the product toyield the end product.

The para-aminobenzoic acid used to illustrate the present invention waspurchased from Aldrich Chemical Company, Inc. and was specified ashaving a molecular weight of 137.14, a melting range of 188° to 189° C.,and a density at its melting point of 1.374. It was classified as being99% pure.

Cobalt p-aminobenzoic acid was prepared by adding p-aminobenzoic acid(137 g, 1 mole) to a solution of sodium hydroxide (40 g, 1 mole) in 900ml. of distilled water at room temperature with mechanical stirring. Tothe resulting brown solution of the sodium salt of p-aminobenzoic acidwas added slowly a solution of cobaltous chloride (120 g, 0.506 mole) in100 ml. of distilled water at room temperature with stirring. A reactiontook place immediately to form a pink precipitate. After the additionwas complete, the reaction mixture was stirred at room temperature foran additional 3 hours. The solid was collected on a filter, washed witha small amount of water and dried at 120° C. under vacuum overnight.After this drying step, dark purple solid cobalt p-aminobenzoic acid wasobtained in 81% yield (135 g).

The rubber compound is described with all ingredients based on the totalrubber component in the compound being 100 parts by weight.

The composition of this invention consists essentially of 100 partsrubber, 40 to 70 phr carbon black, 4 to 10 phr zinc oxide, 10 to 30 phrsilica, 0.5 to 1 phr antioxidant, 0.5 to 1 phr stearic acid, 0.2 to 2.0phr accelerator, 1 to 10 phr hydrocarbon resin, 2 to 6 phr resorcinol, 1to 5 phr Manobond C, 4 to 9 phr sulfur/oil, 80/20, 4 to 9 phr melamineresin and 0.5 to 8 phr of PABA or 2-8 phr of a cobalt salt of PABA.

DETAILED DESCRIPTION OF THE INVENTION

The following Examples are representative of the method and rubbercomposition of this invention. The composition component parts areexpressed in parts per hundred rubber, phr, unless otherwise specified.These components are broadly within the ranges set out below:

    ______________________________________                                        Masterbatch                                                                   ______________________________________                                        Natural rubber         60-80   parts                                          Butadiene              40-20   parts                                          Carbon black           40-70   phr                                            Zinc oxide             4-10    phr                                            Silica                 10-30   phr                                            Stearic acid           .5-1    phr                                            Antiozonant/antioxidant (i.e.,                                                Santoflex 13)          .5-1    phr                                            Hydrocarbon resin      1-10    phr                                            Resorcinol             2-6     phr                                            Manobond C             1-5     phr                                            ______________________________________                                    

    ______________________________________                                        Final Mix                                                                     ______________________________________                                        Accelerator              .2-2   phr                                           Sulfur/oil 80/20         4-9    phr                                           Melamine resin (i.e., Cyrez 963)                                                                       4-9    phr                                           PABA or transition metal salt of PABA                                                                  0.5-8  phr                                           or                                                                            Cobalt PABA (when present)                                                                             2.0-8  phr                                           ______________________________________                                    

Specific detailed examples of useful compositions within the ranges setout above are:

EXAMPLE I

The following composition was Banbury mixed at about 280°-340° F. forseven (7) minutes, or 340° F., whichever occurs first, and a rotor speedof 80 rpm; the resulting masterbatch was then dumped. The Banbury typewas a Type B Internal Mixer (Farrel-Birmingham Company).

    ______________________________________                                                                Parts                                                 ______________________________________                                        (1)     natural rubber        75    parts                                     (2)    butadiene rubber       25    parts                                     (3)    carbon black (FEF)     40    phr                                       (4)    zinc oxide             4.0   phr                                       (5)    pelletized hydrated silica                                                                           10    phr                                       (6)    stearic acid           1.0   phr                                       (7)    N-(1,3-dimethyl butyl)-N'-phenyl-                                                                    1.0   phr                                              p-phenylenediamine                                                     (8)    hydrocarbon resin      5.0   phr                                       (9)    resorcinol (meta-dihydroxybenzene)                                                                   2.5   phr                                        (10)  Manobond C 16          3.5   phr                                       ______________________________________                                    

The hydrocarbon resin was in flake form, had a softening point betweenabout 100° to 110° C., an iodine number of from 125 to 167, and, an ashcontent of 0.05%. The masterbatch resulting from the preceding was thenfinal roll mill mixed for 4 to 8 minutes until dispersion was achievedat a mill speed of approximately 50 rpm and at a temperature of from160° to 180° F.; the resulting final mix was then dumped. Thecomposition components added to the masterbatch prior to final millmixing were as follows:

    ______________________________________                                        (a)    N-tert-butyl-2-benzothiazole                                                                        0.7   phr                                               sulfenamide                                                            (b)    sulfur oil, 80/20     6.0   phr                                        (c)    hexamethoxymethyl-melamine                                                                          4.0   phr                                               powder, and                                                            (d)    PABA.                 1.5   phr                                        ______________________________________                                    

This product was cured for 30 minutes at 300° F., a cure pressure of800-900 psi, and is the invention composition of Table I. The control isthe identical composition without para-aminobenzoic acid.

Table I which follows illustrates the performance of our PABA containingcomposition with bright steel (unplated) wire as it would be utilized.

                  TABLE I                                                         ______________________________________                                                                    Invention                                         Test             Control    Composition                                       ______________________________________                                        R.T. Ring Tensile-                                                            30' Cure/300° F.                                                       300% Modulus, (psi)                                                                            1560       1830                                              Tensile Strength, (psi)                                                                        2340       2410                                              Elongation, (%)   400        410                                              Unconditioned T Adhesion                                                      to (Bright steel)*                                                            30' Cure, R.T. Test                                                                            88 (20)     139 (100)                                        45' Cure, R.T. Test                                                                            89 (20)     137 (100)                                        +' Cure, 230° F. Test                                                                   71 (30)    118 (90)                                          45' Cure, 230° F. Test                                                                  68 (30)    124 (90)                                          Conditioned T Adhesion to                                                     bright steel after 1 hour in                                                  300° F. steam                                                          30' Cure at 300° F.                                                                     61 (10)    118 (90)                                          Tested at 230° F.                                                      ______________________________________                                         (T adhesion values in lb./in.; Coverages in (%) )                             *Bright steelunplated                                                    

EXAMPLE 2

Substantially the same results of Table I are achieved when thefollowing masterbatch and composition added prior to final mill mixingare substituted therein:

    ______________________________________                                        Masterbatch                                                                   ______________________________________                                        (1)  Natural rubber           75     parts                                    (2)  Polybutadiene rubber     25     parts                                    (3)  Carbon black (FEF)       55     phr                                      (4)  Zinc oxide               10     phr                                      (5)  Pelletized hydrated silica                                                                             12.5   phr                                      (6)  Stearic acid             1      phr                                      (7)  N-(1,3-dimethyl butyl)-N'-phenyl-                                                                      1      phr                                           p-phenylenediamine                                                       (8)  Hydrocarbon resin        5      phr                                      (9)  Resorcinol (meta-dihydroxybenzene)                                                                     4      phr                                      (10) Manobond C 16            3.5    phr                                      ______________________________________                                    

    ______________________________________                                        Composition Added to Above Masterbatch                                        ______________________________________                                        (A)     Sulfur/oil - 80/20 powder                                                                          7     phr                                        (B)     Hexamethoxymethyl-melamine                                                                         6     phr                                        (C)     N-tert-butyl-2-benzothiazole-                                                                      0.7   phr                                                sulfenamide                                                           (D)     PABA                 1.5   phr                                        ______________________________________                                    

EXAMPLE 3

A masterbatch identical to that described in Example I was mixed. Thismasterbatch was mixed into a final mix by the procedure described inExample I wherein the following components were added in the final mix:

    ______________________________________                                        (a)    N-tert-butyl-2-benzothiazole-                                                                        0.35  phr                                              sulfenamide                                                            (b)    sulfur oil, 80/20     6.0    phr                                       (c)    hexamethoxymethyl-melamine                                                                          4.0    phr                                              powder, and                                                            (d)    cobalt PABA (Co-PABA) 4.0    phr                                       ______________________________________                                    

This product was cured for 30 minutes at 300° F., a cure pressure of800-900 psi, and is the invention composition of Table II. The controlis the identical composition without cobalt p-amino-benzoic acid.

Table II which follows illustrates the performance of our Co-PABAcontaining composition with bright steel (unplated) wire as it would beutilized.

                  TABLE II                                                        ______________________________________                                                                      Invention                                       Test                 Control  Composition                                     ______________________________________                                        R.T. Ring Tensile - 30' Cure/300° F.                                   300% Modulus, (psi)  1560     1620                                            Tensile Strength, (psi)                                                                            2340     2090                                            Elongation, (%)       400      880                                            Unconditioned T Adhesion to                                                   (bright steel)*                                                               30' Cure, R.T. Test  88 (20)  135 (80)                                        45' Cure, R.T. Test  89 (20)  140 (80)                                        30' Cure, 230° F. Test                                                                      71 (30)  107 (80)                                        45' Cure, 230° F. Test                                                                      68 (30)  102 (80)                                        Conditioned T Adhesion to                                                     bright steel after 1 hour in                                                  300° F. Steam                                                          30' Cure at 300° F.                                                                         61 (10)  108 (90)                                        Tested at 230° F.                                                      ______________________________________                                         (T adhesion values in lb./in.; Coverages in (%) )                             *Bright steelunplated                                                    

EXAMPLE 4

Substantially the same results of Table II are achieved when thefollowing masterbatch and composition added prior to final mill mixingare substituted therein:

    ______________________________________                                        Masterbatch                                                                   ______________________________________                                        (1)  Natural rubber           75     parts                                    (2)  Butadiene rubber         25     parts                                    (3)  Carbon black (FEF)       55     phr                                      (4)  Zinc oxide               10     phr                                      (5)  Pelletized hydrated silica                                                                             12.5   phr                                      (6)  Stearic acid             1      phr                                      (7)  N-(1,3-dimethylbutyl)-N'-phenyl-                                                                       1      phr                                           p-phenylenediamine                                                       (8)  Hydrocarbon resin        5      phr                                      (9)  Resorcinol (meta-dihydroxybenzene)                                                                     4      phr                                      (10) Manobond C 16            3.5    phr                                      ______________________________________                                    

    ______________________________________                                        Composition added to above Masterbatch                                        ______________________________________                                        (A)    Sulfur/oil - 80/20 powder                                                                            7     phr                                       (B)    hexamethoxylmethyl-melamine 6                                                                        phr                                             (C)    N-oxydiethylene benzothiazole-2-                                                                     .8    phr                                              sulfenamide, and                                                       (D)    Cobalt PABA            4     phr                                       ______________________________________                                    

The steel cord adhesion results set forth for bright steel wire(unplated) in the Examples above were determined by the following Tadhesion procedure:

T-ADHESION TEST

1. Using a clicker machine and a 6×1/2 inch die, prepare an adequatenumber of experimental and control stock samples for pad building.

2. Use one piece of calendered fabric backing (0.051").

3. Ply one piece of control rubber stock (0.060") onto the fabricbacking.

4. Place sample in building jig with fabric side down.

5. Place ten cords (of wire) approximately 7" in length equally spacedon top of the two piece assembly.

6. Invert another 2 ply assembly, made as in items 1, 2 and 3 on top ofcords so that cords are between 2 layers of stock to be tested.

7. This assembly should now fit snugly into the mold.

8. Adhesion pads shall be cured for 30 minutes at 300° F., and thenallowed to equilibrate for 24 hours.

9. Testing Machine: 1130 Instron Universal Tester.

10. Test speed 10"/minute; temperature 230° F. after a 20' preheat.

11. The top grip shall be of a special holder made for the cured sample,with a slot in the bottom to permit the sample to be inserted with thewire protruding. The bottom grip shall be a wedge type, designed toexert increasing tightening as the wire is pulled.

12. Record 10 pulls and average. Multiply by 2 to get lbs. adhesion perimbedded inch of wire.

Characteristics of several of the ingredients set out in the examplesare set out below. These definitions are to be considered by way ofillustration and represent known materials that have proven useful inthis invention.

Preferably, the skim stock also contains a suitable proportion of aconventional organo-cobalt complex, such as a material which iscommercially sold under the name "Manobond C". It is known that suchmaterials, including Manobond C, facilitate rubber-to-metal adhesion.

Manobond C is a commercially available source of a cobalt and boroncontaining additive that is compatible in our formulation; it isbelieved to have the structure: ##STR1## wherein each "R" is an alkylradical of from 9 to 12 carbons. Manobond C is available as a blue,viscous liquid; it contains 15.5 to 16.5% cobalt (Manobond C 16) or itcontains 17.5 to 18.5% cobalt (Manobond C 18); it has a viscosity (at25° C.) of 3,000 to 9,000 cps. The ash content is from 22 to 25 weightpercent. Manobond C is commercially available from Wyrough and Loser,Inc., Trenton, N.J.

The rubber to be used in the practice of this invention includesvulcanizable rubbers. Rubbers that can be utilized include naturalrubbers, synthetic rubbers, polyisoprene, polybutadiene, copolymers ofbutadiene and styrene and the like, and blends thereof. The particularrubber composition selected is preferably a blend of natural rubber andpolybutadiene. An extender oil, when utilized, can be, for example, anyknown medium process oil, aromatic or naphthenic hydrocarbon derived.

The antioxidant selected can be, for example, N-(1,3-dimethylbutyl)-N'-phenyl-p-phenylenediamine, known in the trade as Santoflex 13;or other phenyl-p-phenylenediamine derivatives.

The accelerator preferably utilized in the practice of our invention isN-oxydiethylene benzothiazole-2-sulfenamide; this accelerator iscommercially available from American Cyanamid and is known as NOBSSpecial. Other accelerators such asN-t-butyl-2-benzothiazole-sulfenamide can also be utilized; theparticular accelerator selected is not critical.

Any known rubber reinforcing carbon black can be used, such as the knownFEF, ISAF and other carbon blacks. Curing is preferably achieved throughuse of sulfur as a sulfur/oil blend preferably utilized at a ratio of80/20. The use of FEF carbon black is preferred.

The pelletized hydrated silica is commercially available, for example,from PPG, Industries, Inc., Pittsburgh, Pa. The preferred silica isidentified as Hi-Sil 233.

Commercially available hydrocarbon resins that can be utilized in theinvention include, for example, Butaprene 105 (Reichhold Chemical) andPicco 14215 supplied by Hercules, Inc. Included are the intermediate andaliphatic hydrocarbon resins that are otherwise commercially available.The selected hydrocarbon resin will preferably have a softening point offrom about 100° to about 110° C., an iodine number of from about 125 toabout 167 and a maximum ash content of 0.05%.

The melamine resin is preferably Cyrez 963, one of a family of melamineresins marketed by American Cyanamid.

The preceding Examples can be varied within the scope of our totalspecification disclosure, as it would be understood and practiced by oneskilled in the art, to achieve essentially the same results. Equivalentreactants can be used within the designated ranges specified.

Compounding ingredients customarily employed in the rubber compoundingart can be added to our skim stock composition and include accelerators,antioxidants, bactericides and the like, color pigments, extenders,reinforcing pigments, softeners, vulcanizing agents, etc. Thecompounding ingredients are used in the amounts necessary to achieve thedesired properties in the resulting vulcanizate as is well known tothose skilled in the art.

The skim stock of the present invention can be applied by use ofcalendering means, spray means or other known application techniques.Areas of significant utility include, but are not limited to, radiatorhose, pneumatic tires, air ride springs, metal reinforced products suchas rubber bumpers and sporting goods grips such as golf club handles; ineach of these representative areas of utility, the skim stockcomposition can be used to increase adhesion and adhesion retentionproperties between metal and rubber, including use in operation whenbright steel surfaces are present.

When the skim stock of this invention is used in steel cord tireconstruction, for example, it is extremely important, both in new tireconstruction and retread or repair operations, that the bond between therubber ply stock and the wire fabric be as flexible and as strong aspossible for efficient use under operation conditions; this isespecially important in the case of truck tires which are subjected tohigh loads and speeds with consequent heat buildup due to the rapidflexing of the plies.

The present invention also finds utility in, for example, metal-rubberarticles such as motor mounts, cutless bearings, torsilastic springs,power belts, printing rolls, metal wire reinforced or braided hose,electrical deicers, shoe heels, and wherever it is desired to securerubber to plated or unplated metal to provide a flexible and strong bondbetween the same.

Acceptable results would be achieved on substituting brass or zincplated steel wire for the bright steel wire of Tables I and II. The wirecoated in the practice of our invention can be, for example, brassplated wire, i.e., 70% Cu, 30% Zn, zinc plated, or, bright (unplated)steel. The wire can be in the form of a strand, mat, web, ply or braid.

We claim:
 1. In a cured rubber skim stock of conventional composition,said stock being bonded to at least one metal member contained withinsaid stock, the improvement wherein a minor proportion ofpara-aminobenzoic acid or a cobalt salt of para-aminobenzoic acid isincorporated into said stock prior to curing.
 2. The composition ofclaim 1 having 0.5 to 8.0 phr of para-aminobenzoic acid.
 3. Thecomposition of claim 1 having 2.0 to 8.0 phr of a cobalt salt ofpara-aminobenzoic acid.
 4. A composition according to claim 1 whereinsaid curing is achieved by vulcanization.
 5. In a method for improvingthe adhesion of a cured rubber skim stock of conventional composition toat least one metal member contained within said stock, the improvementwherein a minor proportion of para-aminobenzoic acid or a cobalt salt ofpara-aminobenzoic acid is incorporated into said stock prior to curing.6. The method of claim 5 wherein from 0.5 to 8.0 phr ofpara-aminobenzoic acid is incorporated into said stock prior to curing.7. The method of claim 5 wherein from 2.0 to 8.0 phr of a cobalt salt ofpara-aminobenzoic acid is incorporated into said stock prior to curing.8. The method of claim 4 wherein said curing is achieved byvulcanization.
 9. A tire having improved adhesion between a cured rubberskim stock of conventional composition, and, metal reinforcing membersimbedded therein, the improvement wherein a minor proportion ofpara-aminobenzoic acid or a cobalt salt of para-aminobenzoic acid hasbeen incorporated into said stock prior to curing.
 10. A tire accordingto claim 9 wherein said curing is achieved by vulcanization.
 11. Thetire of claim 9 wherein said composition contains from 0.5 to 8.0 phr ofpara-aminobenzoic acid.
 12. The tire of claim 9 wherein said compositioncontains 2.0 to 8.0 phr of a cobalt salt of para-aminobenzoic acid.